Plasma wave field effect transistor as a resonant detector for 1 terahertz imaging applications

The possibility of using plasma wave field effect transistor in a time domain terahertz (THz) spectroscopy setup is presented. We demonstrate that High Electron Mobility Transistors (HEMTs) is an efficient device for detection of pulsed terahertz electric fields generated with a femtosecond laser oscillator. The response was observed in the frequency range of about 1 THz, far above the cutoff frequency of the transistors at room temperature. We show that the physical mechanism of the detection is related to the plasma waves excited in the transistor channel and that significant improvement of the active device can be achieved by increasing the drain current. The two-dimensional terahertz imaging applications clearly demonstrate that plasma wave nanometer HEMT should be employed as efficient future detectors in a matrix configuration.     

Milliwatt-level mid-infrared (10.5-16.5 μm) difference frequency generation with a femtosecond dual-signal-wavelength optical parametric oscillator

We demonstrate the generation of mid-infrared radiation using a femtosecond dual-signal-wavelength optical parametric oscillator and difference frequency generation in an extracavity gallium selenide or silver gallium diselenide crystal. This system generates up to 4.3?mW of average mid-infrared power. Its spectra can be tuned to between 10.5?μm and 16.5?μm wavelength (952 cm^-1-606 cm^-1) with more than 50 cm^-1 spectral bandwidth. We demonstrate that the power and spectra of this system are temporally very stable.     

Femtosecond pulse-shape modulation at nanosecond rates

We report high-rate, computer-controlled femtosecond pulse shaping by use of an electro-optical gallium arsenide optical phased-array modulator with 2304 controlled waveguides. It provides fast modulation speed of both spectral phases and amplitudes. Limited by the driving electronics of our current setup, we were able to update a pulse shape in approximately 30 ns. This technique paves the way toward individual shaping of every single pulse of typical femtosecond mode-locked oscillators.     

Multiple four-wave mixing in optical fibers: 1.5-3.4-THz femtosecond pulse sources and real-time monitoring of a 20-GHz picosecond source

In this work, we report recent progress on the design of all-fibered ultra-high repetition-rate pulse sources for telecommunication applications around 1550 nm. The sources are based on the non-linear compression of an initial beat-signal through a multiple four-wave mixing process taking place into an optical fiber. We experimentally demonstrate real-time monitoring of a 20 GHz pulse source having an integrated phase noise 0.01 radian by phase locking the initial beat note against a reference RF oscillator. Based on this technique, we also experimentally demonstrate a well-separated high-quality 110 fs pulse source having a repetition rate of 2 THz. Finally, we show that with only 1.4 m of standard single mode fiber, we can achieve a twofold increase of the repetition rate, up to 3.4 THz, through the self-imaging Talbot effect. Experimental results are supported by numerical simulations based on the generalized non-linear Schrödinger equation.     

Widely linear and non-phase-matched optical-to-terahertz conversion on GaSe:Te crystals

We demonstrate the widely linear and broadband terahertz (THz) generation on GaSe:Te crystals by femtosecond laser pulses. It was found that the dopant, Te atoms, in GaSe crystals significantly enhances the efficiency of THz generation, and its central frequency can be tuned by varying the crystal thickness through non-phase-matched optical rectification. Moreover, the wide-ranging linearity for the optical-to-THz conversion and central-frequency-tunable THz generation promise for GaSe:Te crystals to be potential materials for high-power (>1.36 μW) THz applications.     

Megahertz optical parametric amplifier pumped by a femtosecond oscillator

We demonstrate, for the first time to our knowledge, an optical parametric amplifier directly pumped by a femtosecond oscillator. Wavelength-tunable pulses in the ranges 0.65-0.85 microm (signal) and 1.4-2.5 microm (idler) are generated at a repetition frequency of 1 MHz. For pumping the beta-barium borate crystal we use a microjoule Yb:KY(WO4)2 femtosecond oscillator with cavity dumping. Pulses with 30 nJ of energy and a duration of 16 fs are achieved from a supercontinuum seed generated in a sapphire plate.     

Generation of widely tunable Fourier-transform-limited terahertz pulses using narrowband near-infrared laser radiation

Widely tunable, Fourier-transform-limited pulses of terahertz (THz) radiation have been generated using (i) crystals of the highly nonlinear organic salt 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST), (ii) zinc telluride (ZnTe) crystals, (iii) gallium phosphide (GaP) crystals, and (iv) low-temperature-grown gallium arsenide (LTG-GaAs) photomixers with THz spiral antennas. Outputs from two narrowband (Δν < 1 MHz, λ ∼ 800 nm) cw titanium-doped sapphire (Ti:Sa) ring lasers with a well-controlled frequency difference were shaped into pulses using acousto-optic modulators (AOM), coupled into an optical fiber, pulse amplified in Nd:YAG-pumped Ti:Sa crystals and used as optical sources to pump the THz emitters. The THz radiation was detected over a broad frequency range and its bandwidth was determined to be ∼10 MHz. The spectroscopic potential of the THz source is illustrated by the absorption spectrum of a pure rotational transition of OCS.     

The dependence on optical energy of terahertz emission from air plasma induced by two-color femtosecond laser-pulses

The generation of terahertz （THz） emission from air plasma induced by two-color femtosecond laser pulses is studied on the basis of a transient photocurrent model. While the gas is ionized by the two-color femtosecond laser-pulses com- posed of the fundamental and its second harmonic, a non-vanishing directional photoelectron current emerges, radiating a THz electromagnetic pulse. The gas ionization processes at three different laser-pulse energies are simulated, and the corresponding THz waveforms and spectra are plotted. The results demonstrate that, by keeping the laser-pulse width and the relative phase between two pulses invariant when the laser energy is at a moderate value, the emitted THz fields are significantly enhanced with a near-linear dependence on the optical energy.     

Multiplying the repetition rate of passive mode-locked femtosecond lasers by an intracavity flat surface with low reflectivity

By inserting a low-reflectivity flat surface inside the oscillator cavity, we demonstrate a flexible and phase-insensitive method for multiplying the repetition rate of a femtosecond passive mode-locked solid-state laser. Without mode matching and feedback control, we successfully multiplied the repetition rate of a passively mode-locked Cr:forsterite laser from 124 MHz to 1.24 GHz. High-repetition-rate femtosecond optical pulses with average power of >100 mW can be obtained with the demonstrated method.     

Narrowband and tunable ring optical parametric oscillator with a volume Bragg grating

We demonstrate a tunable nanosecond optical parametric oscillator with a narrowed signal spectrum. This was done by use of a volume Bragg grating based retroreflector, which makes the tuning simple and yields a compact design. Using periodically poled KTiOPO4 as the nonlinear medium, we generated 0.42 mJ of signal energy at 760 nm with a tuning range of 5 nm(2.6 THz) and a bandwidth of 0.25 nm(130 GHz) when the oscillator was pumped at 532 nm with 1.3 mJ of energy.     

THz generation via optical rectification with ultrashort laser pulse focused to a line

We report on efficient THz pulse generation via optical rectification with femtosecond laser pulses focused to a line by a cylindrical lens. This configuration provides phase-matched conditions in the superluminal regime. 35 pJ THz pulses have been generated with this technique in a stoichiometric LiNbO₃ crystal pumped by 2 μJ femtosecond laser pulses at room temperature. An unusual superquadratic rise of the THz pulse energy with the laser pulse energy has been observed at high laser energies. This extraordinary energy dependence of the THz generation efficiency is explained by self-focusing of the laser beam in the crystal. Z-scan measurements and comparison of the THz pulse spectra created with laser pulses having different energies confirm this interpretation.     

Frequency dependent optical and dielectric properties of zinc sulfide in Terahertz regime

Frequency dependent optical and dielectric properties for several grades of chemical vapor deposited (CVD) zinc sulfide (standard, elemental, and multi-spectral) was performed using a terahertz time-domain spectroscopy (THz-TDS) system in the frequency range from 0.15 THz to 2.5 THz. Zinc sulfide exhibits low frequency vibrational modes characterized by the THz-TDS. Two low-frequency phonon resonance lines were revealed at 0.78 THz and 2.20 THz. These samples were also characterized in the GHz range using a backward wave oscillator (BWO) source quasi-optical spectrometer, and the data obtained by both approaches were compared. Experimental data were also compared with an undamped harmonic oscillator model. These results compare well with the literature values obtained using other methods.     

Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb

We demonstrate a great simplification in the long-standing problem of measuring optical frequencies in terms of the cesium primary standard. An air-silica microstructure optical fiber broadens the frequency comb of a femtosecond laser to span the optical octave from 1064 to 532 nm, enabling us to measure the 282 THz frequency of an iodine-stabilized Nd:YAG laser directly in terms of the microwave frequency that controls the comb spacing. Additional measurements of established optical frequencies at 633 and 778 nm using the same femtosecond comb confirm the accepted uncertainties for these standards.     

超快太赫兹时域光谱系统

超快太赫兹时域光谱系统是基于高速异步光学采样原理进行工作的,该系统使用2个重复频率可在1 GHz附近变化的飞秒振荡器,并使用高带宽反馈电路控制其重复频率。2个飞秒振荡器的重复频率存在Δf的失谐,一个飞秒振荡器的重复频率是1 GHz+Δf Hz,为泵浦脉冲;另一个飞秒振荡器的重复频率是1 GHz,为探测脉冲,由此提供泵浦脉冲和探针脉冲的时间差,时间延迟呈周期性变化,其扫描周期可以由1/Δf给出。此系统摒弃了传统THz-TDS系统所必需的机械延迟线,采用双光子探测器来产生触发信号。当设定Δf=1 kHz时,1 ms就可以探测出1个THz谱, 用时10.3 s即可得到动态范围为21 dB、频谱分辨率为5 GHz的太赫兹信号。该系统具有检测速度快和频谱分辨率高的优点,在需要快速测量的应用环境中有着传统太赫兹时域光谱系统不可比拟的优势。     

Ultrafast control of slow light in THz electromagnetically induced transparency metasurfaces

In this paper,we experimentally demonstrate ultrafast optical control of slow light in the terahertz(THz) range by combining the electromagnetically induced transparency(EIT) metasurfaces with the cut wire made of P~+-implanted silicon with short carrier lifetime.Employing the optical-pump THz-probe spectroscopy,we observed that the device transited from a state with a slow light effect to a state without a slow light effect in an ultrafast time of 5 ps and recovered within 200 ps.A coupled oscillator model is utilized to explain the origin of controllability.The experimental results agree very well with the simulated and theoretical results.These EIT metasurfaces have the potential to be used as an ultrafast THz optical delay device.     

飞秒脉冲正交位相压缩光的产生

利用锁模飞秒脉冲激光二次谐波为抽运源, 同步抽运单共振光学参量振荡器, 抽运光中心波长为425 nm, 重复率为76 MHz, 脉宽180 fs, 光学振荡器下转换晶体采用Ⅰ类共线PPKTP, 实验上实现了压缩度为2.58 dB的正交位相压缩光.考虑到实验系统的效率, 可以推知光学参量振荡器输出的下转换光压缩度为 4.48 dB. 关键词： 同步抽运光学参量振荡器 压缩光 锁模飞秒脉冲     

Melting behaviours of nickel nanorods

We report the synthesis and measurement of an ultra-precise and extremely stable optical frequency in the telecommunications window around 1543 nm. Using a fibre-based femtosecond frequency comb we have phase-stabilised a fibre laser at 194 THz to an optical frequency standard at 344 THz, thus transferring the properties of the optical frequency standard to another spectral region. Relative to the optical frequency standard, the synthesised frequency at 194 THz is determined to within 1 mHz and its fractional frequency instability is measured to be less than 2×10^-15 at 1 s, reaching 5× 10^-18 after 8000 s. We also measured the synthesised frequency against a caesium fountain clock: here the frequency comparison itself contributes less than 4 mHz (2×10^-17) to the uncertainty. Our results confirm the suitability of fibre based frequency comb technology for precision measurements and frequency synthesis, and enable long-distance comparison of optical clocks by using optical fibres to transmit the frequency information.     

Highly sensitive single-beam heterodyne coherent anti-Stokes Raman scattering

Single-beam coherent anti-Stokes Raman-scattering (CARS) microspectroscopy achieves a complete CARS scheme with a femtosecond laser. Here, we introduce heterodyne detection in a simple experimental extension: the optical fields driving the CARS process and the local oscillator used for heterodyning are derived from a single beam of ultrashort laser pulses by pulse shaping. The heterodyne signal is amplified by more than 3 orders of magnitude and is linearly dependent on the concentration of Raman scatterers. This dramatically increases the sensitivity of chemically selective detection at microscopic resolution while maintaining the simplicity of the single-beam setup.     

Narrow-band terahertz wave generation and detection in one periodically poled lithium niobate crystal

In this article, we present studies on therahertz (THz) wave generation and frequency up-conversion in a periodically poled lithium niobate (PPLN) crystal. A frequency at 1.37 THz was generated as femtosecond pump pulses passed through a PPLN crystal with grating periods of 30 μm. The pump-induced THz wave interacts with the probe wave in the crystal by frequency mixing. The frequency up-converted THz wave is easily detected by a normal photodiode. A new scheme for generation and detection of THz wave in one non-linear crystal was proposed.     

Compact and high-power broadband terahertz source based on femtosecond photonic crystal fiber amplifier

We present a review of the development of a compact and high-power broadband terahertz (THz) source optically excited by a femtosecond photonic crystal fiber (PCF) amplifier.The large mode area of the PCF and the stretcher-free configuration make the pump source compact and very efficient.Broadband THz pulses of 150 μW extending from 0.1 to 3.5 THz are generated from a 3-mm-thick GaP crystal through optical rectification of 12-W pump pulses with duration of 66 fs and a repetition rate of 52 MHz.A strong saturation effect is observed,which is attributed to pump pulse absorption;a Z-scan measurement shows that three-photon absorption dominates the nonlinear absorption when the crystal is pumped by femtosecond pulses at 1 040 nm.A further scale-up of the THz source power is expected to find important applications in THz nonlinear optics and nonlinear THz spectroscopy.